Abstract

Distal tephra beds provide important records of pyroclastic volcanism that enhance our overall understanding of eruptive frequencies, magnitudes, compositions, and hazards. Some beds also serve as widespread chronostratigraphic markers. Lacustrine sediments near Summer Lake, Oregon (United States), record numerous eruptions of Cascade arc sources over a period exceeding 2.5 m.y. Late Pleistocene sediments exposed in outcrop have yielded 88 visible tephra beds, including many beds not previously documented. Of these beds, 44 are characterized by rhyolitic glass, 40 contain predominantly basaltic or intermediate glass, and 4 are strongly heterogeneous in composition. Only 23 have been correlated to deposits outside of the Summer Lake basin. The remaining 65 beds provide a record of Cascade arc volcanism that is as yet unique to Summer Lake. Age-depth relations are well constrained for the upper 6 m of section, but are less certain in the lower 12.4 m. Tephra correlations and an overall age model suggest the following: bed B1 originates from an eruption of Mount Mazama (Crater Lake) ca. 20 ka. Beds I and W likely originate from eruptions of Mount St. Helens ca. 80 and 190 ka. A 7-cm-thick tephra bed correlated to Shevlin Park Tuff probably dates to ca. 198 ka. Tephra correlated to the Antelope Well tuff from Medicine Lake volcano dates to ca. 215 ka. Bed NN, at the base of the section, has an estimated age of at least 240–250 ka and probably originated from Newberry Volcano. Overall, this record significantly refines the Pleistocene tephrostratigraphic framework for western North America.

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